1. Field of the Invention
This invention relates to catalytic cracking of petroleum fractions. More particularly, this invention relates to an improved process, and the apparatus used therein, for converting gas oils and heavy petroleum fractions into valuable hydrocarbon products, such as gas, gasoline, light cycle gas oil and heavy cycle gas oil, with reduced coke formation in a fluid catalytic cracking reactor.
2. Description of Prior Art
Conversion of various petroleum fractions to more valuable products in catalytic reactors is well known in the art. The petroleum industry has found the use of a fluid bed catalytic cracker reactor (hereinafter FCC reactor) particularly advantageous for that purpose. An FCC reactor typically comprises a thermally balanced assembly of apparatus comprising a reactor vessel filled with a catalyst and a regenerator vessel wherein spent catalyst is regenerated. The feed is converted in the reactor vessel over the catalyst and coke simultaneously forms on the catalyst, thereby deactivating the same. The deactivated (spent) catalyst is removed from the reactor vessel and conducted to the regenerator vessel, wherein coke is burned off the catalyst with air, thereby regenerating the catalyst. The regenerated catalyst is then recycled to the reactor vessel. The reactor-regenerator vessel assembly must be maintained in steady state heat balance so that heat generated by burning the coke provides sufficient thermal energy for catalytic cracking in the reactor vessel. The steady-state heat balance is usually achieved and maintained in the FCC reactors by controlling the rate of flow of the regenerated catalyst from the regenerator to the reactor. The rate of catalyst flow is normally controlled by means of a slide valve in the regenerator-to-reactor conduit. The degree of opening of the slide valve is controlled by a conventional controlling means coupled to a temperature sensing means (e.g., a thermocouple), placed at the top of the reactor, to maintain the desired temperature inside the reactor.
The product stream of the catalytic cracker is usually fractionated into a series of products, including: gas, normally conducted to gas concentration plant; gasoline; light cycle gas oil; and heavy cycle gas oil. A portion of the heavy cycle gas oil is usually recycled into the reactor vessel and mixed with fresh feed. The bottom effluent of the fractionator is conventionally subjected to settling and the solid portion of the settled product is also recycled to the reactor vessel in admixture with the heavy cycle gas oil and feed.
In a prior art version of fluid catalytic cracking reactor, the regenerated catalyst is introduced into the base of a riser column (also known as transport reactor) in the reactor vessel. The riser column serves a two-fold purpose: (1) to transfer the catalyst from the regenerator to the reactor, and (2) to initiate cracking of the petroleum feed. The regenerated hot catalyst is admixed in the bottom of the riser column with a stream of fresh feed and recycled petroleum fractions, and the mixture is forced upwardly through the column. During the upward passage of the catalyst and of the petroleum fractions, the petroleum is cracked and reformed, and coke is simultaneously deposited on the catalyst. The fluid bed of the coked catalyst and of the cracked and reformed petroleum components is passed upwardly out of the riser and through a solid-gas separation system, e.g., a series of cyclones, at the top of the reactor. The cracked petroleum fraction is conducted to product separation, while the choked catalyst passes to the regenerator vessel and is regenerated therein, as discussed above.
In the prior art design of the riser (see e.g., Pfeiffer, U.S. Pat. No. 3,492,221; Pfeiffer, U.S. Pat. No. 3,607,126 and Schwarzenbek, U.S. Pat. No. 3,565,790), the petroleum feed must be contacted with the catalyst for a relatively long period of time to effect efficient conversion and cracking of the feed. The relaively long contact time is necessitated by the upflowing configuration of the riser which requires acceleration of the catalyst from an essentially stationary position to the feed velocity against the downward force of gravity. This causes commonly encountered problems in the upflow reactor riser, such as backmixing of the catalyst and non-uniform distribution thereof through the feed.
In addition, some petroleum feeds, e.g., gas oils and hydrotreating (HDT) vacuum residues, have a natural tendency to form high quantities of coke, thereby sacrificing the yield of gasoline-grade products. This tendency leads to time consuming and sometimes inefficient operation of the regenerating vessel and, in extreme cases, to an upset of the steady-state heat balance equilibrium. Due to relatively long contact time and inefficient contacting of catalyst and petroleum feeds in the prior art upflow reactor riser, the tendency of the aforementioned petroleum feeds to form excessive amounts of coke at the expense of gasoline-grade products is even more pronounced. Further details of FCC processes can be found in U.S. Pat. Nos. 2,383,636 (Wurth); 2,689,210 (Leffer); 3,338,821 (Moyer et al); 3,812,029 (Snyder, Jr.); 4,093,537 (Gross et al); 4,118,337 (Gross et al); and 4,118,338 (Gross et al); as well as in Venuto et al, Fluid Catalytic Cracking with Zeolite Catalysts, Marcel Dekher, Inc. (1979). The entire contents of all of the above patents and publications are incorporated herein by reference.
Accordingly, it is a primary object of the present invention to decrease contact time of catalyst and petroleum feed, thereby improving the efficiency of contact of the catalyst and the feed.
Another object of this invention is to effect a rapid separation of the catalyst from the feed after the completion of the reaction in the riser.
It is an additional object of this invention to improve conversion-coke selectivity of the FCC plant, thereby decreasing the relative amount of coke produced in the plant.
It is yet another object of this invention to increase catalyst efficiency and increase gasoline selectivity in the FCC plant.
It is yet another object of this invention to improve octane rating of the gasoline produced in the FCC plant.
Another object of this invention is to provide an improved apparatus for the FCC plant wherein the riser of the reactor vessel is placed on top of the reactor vessel in such a manner that it forces the downflow movement of the regenerated catalyst mixed with the petroleum feedstock.
Additional objects and advantages of this invention will become apparent to those skilled in the art from the study of the specification and of the appended claims.